Of course it wont perfom as well (not in a bad way), you will still have some thermal handling and mechanical handling left in the sub which can be a good thing (an advantage in case you ever want to upgrade). There are no disadvantages of running less than the subs rms to the sub, only if your trying to make your amp put out more power then it can and you start sending dirty signals to the sub. I just put a Sundown Z v.1 15" sub on the same Infinity amp you have (a 1000 watt rms sub) and it freaking pounds like no other, wishing I would of keep the sub for myself. Just a heads up that there is no such think as underpowering, you could always just make your box a little bigger than recommended if you have the room to take up for the low amount of power being used (just another option).